Rotary Seal Design Guide - Seals Unlimited

Catalog EPS 5350/USAEngineeringThis also means that the primary function ofsingle spring-loaded designs is dependent on theinstallation direction. While the seal will performboth retention and exclusion functions, they arenot performed equally.If the primary function is retention, the sealshould be installed with the steep lip angle facingtowards the lubricant. This is normally the openfacedside. If the primary function of the seal is toexclude, the steep angle needs to face toward thecontaminant (see Figure 2-3).Several spring types are used to energize thelip. The most common is a wound spring, oftenreferred to as a garter spring. Finger springs areanother option, although their loading is typicallyless uniform and they can be subject to severedistortion prior to or during installation, leading toareas of the lip that are not properly loaded. Otherspring types used are cantilever, canted-coil andhelical which are normally used in PTFE designs.In order for the spring to maintain the proper loadover the life of the seal, the spring must becompatible with the fluids and the temperature ofthe application.The dimensional relationship between thecenter of the spring and the lip contact point iscalled the R value. The leading edge of the lipshould be toward the oil side, with the centerlineof the spring slightly toward the air side. If thecenterline of the spring is too far toward the airside (too positive R value) it will put too much ofthe lip (wide footprint) in contact with the shaft andcause excessive wear. A spring position that is tooclose to the lip contact point (negative R value)can cause the lip to become unstable or roll anddump the spring.A spring-energized lip is required for positiveoil retention, but not typically for grease retention(see Figure 2-4).22Figure 2-3. Installations Facing Lubricant andContaminantIf both retention and exclusion are critical andthe level of contaminants is heavy, one seal shouldbe used to retain the lubricant, and exclusioncapacity should be added using another lip seal,auxiliary excluders or by upgrading to a bearingisolator (see Page 2-27).The purpose of the spring is to provide aconstant, uniform load of the lip on the shaft for thelife of the seal. The spring keeps the seal lip incontact with the shaft during higher shaft speedsand also overcomes compression set and wear ofthe lip material. Compression set of the lipmaterial is normal as it is subjected to thermalcycles during operation.Oil SealFigure 2-4. Oil and Grease SealsGrease SealThe rotary shaft seal is only one component inthe sealing system. There are several keyoperating parameters that can work in unison tooptimize seal life, or conversely, if misapplied,can reduce seal life to a few operational hours.03/28/062-5 Parker Hannifin CorporationEPS DivisionToll Free: (800) 233-3900www.parkerseals.com